1. Field of the Invention
This invention relates to a clock switching circuit that switches between first and second clocks that are not synchronized with each other and have different frequencies.
2. Description of the Related Art
A conventional clock switching circuit switches between a first clock CLK1 and a second clock CLK2 simply in response to a clock selection signal SEL, as shown in FIG. 6. Japanese Patent Application Publication Nos. H07-248843, 2003-223237, 2003-347931, 2004-54350 and 2005-50327 disclose various clock switching circuits.
With the clock switching circuit which switches between the first clock CLK1 and the second clock CLK2 simply in response to the clock selection signal SEL, however, there arises a hazard, that is, an unwanted pulse, and a distortion of a duty ratio of the clock when the first clock CLK1 and the second clock CLK2 are not in synchronization with each other and have different frequencies, resulting in malfunctioning of a circuit that operates with the clocks. For example, when the circuit that operates with the clocks is a flip-flop, the flip-flop may fall into a metastable state because of irrelevant setup/hold or the like.